This invention relates to a new and improved method and system for transmitting an analog signal such as a signal containing information in the microwave spectrum using fiber optics to a utilization device or devices. The method and system of the invention provides for much increased performance in the dynamic range of analog information which may be transmitted from a source of analog signals to a remotely located utilization device.
Conventional coaxial cable systems for transmitting a signal from, e.g., an antenna, to, e.g., a receiver, inherently are subject to large line losses when transmitting analog information at microwave and rf frequencies. A typical loss figure for a conventional coaxial system is 600 dB per 100 meters at 10 GHz. The distance between the analog signal source and receiver or receivers must, therefore, be kept small or appropriate amplification must be provided. These restrictions effectively prevent the use of coaxial systems for long-distance transmission purposes.
Optical fiber systems have also been used to transmit high frequency analog signals and they offer the significant advantage of low line loss over cable systems. A typical loss figure for an optical fiber system is 0.5 dB per 1000 meters, and the maximum usable distance between the signal source and receiver may be as high as 10 to 30 kilometers without the necessity for amplification. Another significant advantage offered by an optical fiber system is that the costs are much lower than those for a coaxial cable system.
The usual approach to the transmission of analog signals using a fiber optic system is to use the microwave signal to intensity modulate light from a source of coherent light such as a laser. These systems or approaches, however, involve a severe limitation on the dynamic range of the system. The noise floor is limited by the intensity noise created by the source, about -130 dB/.sqroot.Hz to the laser output for a typical device. Normalized to a 1 MHz bandwidth, this is equivalent to a -70 dB noise floor related to the laser output. Thus, if 100% intensity modulation depth is induced only 70 dB of dynamic range is achievable. Accordingly, fiber optic systems signal transmission of the kind referred to are limited in the dynamic range of high frequency signal information they are capable of transmitting to a utilization device.